Black-colored films are used in various fields, such as accessories, due to their excellent appearance. They are also used in various optical instrument parts, measuring instrument parts, etc., by taking advantage of their low reflectivity. Various films, including chromium-, nickel-, zinc-, aluminum-, and iron-based films, are known as black-colored films. Various methods, including chemical treatment and anodic oxidation, are known as the method for forming such films.
In particular, black chromium plating is widely used as the method for forming black films (see PTL 1, PTL 2, and PTL 3, listed below). Plating baths used in these black chromium plating methods contain hexavalent chromium ions; however, the toxicity of hexavalent chromium has recently become an issue. In terms of the working environment and waste water treatment, there is a strong demand for the development of plating solutions with low toxicity. Therefore, black plating from a trivalent chromium bath with low toxicity has been spreading as an alternative technology (see PTL 4 and PTL 5, listed below).
However, compared to black plating films obtained from a hexavalent chromium bath, the color of films obtained from a trivalent chromium plating bath is less blackish, and is a dark brown that is close to black. More blackish films are demanded in terms of decorativeness.